Methods and systems for capacitive motion sensing and position control

ABSTRACT

A system for detecting motion and proximity by determining capacitance between a sensor and an object. The sensor includes sensing surfaces made of a thin film of electrically conductive material mounted on a non-conductive surface. In another embodiment, the sensor is a human body. The sensor senses the capacitance between a sensor&#39;s surface and an object in its vicinity and provides the capacitance to a control system that directs machine movement. Because the sensor does not require direct contact or line-of-sight with the object, a machine can be controlled before harm occurs to the object.

This application is a divisional of U.S. application Ser. No.09/678,916, filed Oct. 4, 2000, now U.S. Pat. No. 6,661,240, which ishereby incorporated by reference.

BACKGROUND OF THE INVENTION

This invention relates generally to devices used to provide safety forhumans in proximity with moving equipment, and more specifically tomotion and proximity sensors employed as part of a control system toorient equipment based on capacitance.

Safety is important when people are close to moving machines. One suchexample is locally controlled machines or robotic equipment where peopleare in close proximity to moving mechanical components. Another exampleis in the medical imaging equipment industry.

In known systems, conventional safety mechanisms such as mechanicalswitches and fluid-filled bladders connected to pressure switches aretypically mounted directly to the moving mechanical components, or inproximity of the hazardous area. These conventional safety mechanismsrequire direct contact between the person or inanimate object and thesafety mechanism to operate. For example, the fluid-filled bladdermounted to a moving mechanical component uses a pressure sensor or apressure switch inside the bladder to detect increased pressure as thebladder makes contact with an object. The sensed pressure increasetypically is an input to a control system which stops the movingmechanical component.

In other known systems, plates, levers, cables, and rings are connectedto mechanical switches and mounted on the moving mechanical component.The switches are activated when the plate, lever, cables, or ringcontacts the person or object, and the machine is stopped before anyharm occurs.

Disadvantages of the above described systems include expense(fluid-filled bladders) and the fact that the sensing area is highlylocalized (mechanical switches). Such devices are typically ON or OFFand therefore provide no information to the control system regardingrelative distance between the subject and the sensor. systems. Thedrawback to those systems is that an unobstructed line-of-sight betweenthe detector and the subject is required. As applied to medical imagingequipment, required sterile covers and drapes preclude use ofline-of-sight proximity detector systems. Depending on theimplementation specifics, these sensors are also highly directional andimpacted by object properties such as reflectivity and specularity,which further limits their applicability.

In the listed examples, safety cannot be enhanced, nor injury preventedsimply by increasing the distances between man and machine because eachexample requires close proximity between man and machine. It wouldtherefore be desirable to provide a system whereby proximity andrelative distance to a person or an object can be sensed and theinformation regarding proximity and distance used to control movementand prevent contact with the person or object and thereby increase thesafety of such a system.

BRIEF SUMMARY OF THE INVENTION

In one aspect, the present invention relates to a method and system fordetecting motion using a capacitance between a sensor and an object.Alternatively, the invention detects proximity of an object using thecapacitance between a sensor and the object. In an exemplary embodiment,a capacitance based proximity sensor is used as a detector. The sensorincludes sensing surfaces made of a thin film of electrically conductivematerial mounted on a non-conductive surface. The non-conductive surfacecan take any shape and form. The sensor senses the capacitance between aconductive surface and an object placed in its vicinity, and the sensorprovides a capacitance value to a control system. The control system isprogrammed to use the capacitance data to control the movement of amachine or piece of equipment. In one embodiment, the piece of equipmentis a medical imaging system.

In another embodiment, the sensing surface is a human body. A relativecapacitance between the human body and surrounding objects isdetermined. The control system uses the capacitance information todetermine a position of the body and proximity of objects near the bodyto control movement of a machine or piece of equipment.

Accordingly, because the sensor can take any size and shape, and doesnot require direct contact or line-of-sight with the object to determineif an object has moved, a machine or piece of equipment can becontrolled before harm occurs to the object.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a system for detecting capacitance changebased upon movement;

FIG. 2 is a diagram showing an alternative system for detectingcapacitance based upon movement;

FIG. 3 is a diagram showing one embodiment of a capacitance basedproximity sensor;

FIG. 4 is a diagram showing an alternative embodiment of a capacitancebased proximity sensor;

FIG. 5 is a diagram of a third embodiment of a capacitance basedproximity sensor;

FIG. 6 is a diagram of a fourth embodiment of a capacitance basedproximity sensor;

FIG. 7 is a diagram of a sensing field for a capacitance based proximitysensor;

FIG. 8 is a diagram of a sensing field for a capacitance based proximitysensor shaped by sensor surface geometry;

FIG. 9 is a diagram of a medical imaging system using capacitive basedproximity sensors; and

FIG. 10 is an illustration of an irregularly shaped apparatus with anouter surface covered with sensing material.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a diagram showing a system 10 for detecting capacitance changebased upon movement of, or proximity to, an object 12. Object 12 isconnected to a capacitance sensing circuit 14 via a conductive strap 16.Sensing circuit 14 senses capacitance and supplies data relating to themeasured capacitance to control system 18. Object 12 is on a surface 20which includes a non-conductive surface 22 such as a film or mat placedupon a conductive surface 24. Control system 18 is programmed to use themeasured capacitance data to control movement of component 26 in bothhorizontal and vertical axes via a motor 28. Component 26 in oneembodiment is a radiation source. Component 26 in an alternativeembodiment is a detector. Component 26 in a further alternate embodimentis a sensor. Component 26 in a still further embodiment is a nuclearmedicine imaging source. Component 26 in another embodiment is a lasersource. Component 26 in yet another embodiment is a component of amedical system, e.g., computer aided tomography (CAT), magneticresonance imaging (MRI), computed tomography (CT), digital fluoroscopy,positron emission tomography (PET), positron emission transaxialtomography (PETT), and mammography.

The amount of capacitance sensed by circuit 14 changes as object 12moves. In another embodiment, the capacitance sensed also changes as aproximity of object 12 changes with respect to component 26. The changein capacitance is received by control system 18 which in turn causeschanges in predetermined movement of component 26 such that thetrajectory of object 26 is optimized for a procedure being performed. Anability to detect unexpected motion of object 12 provides control system18 or an operator of control system 18 with a signal to slow or stopmovement of component 26 to prevent injury to object 12 or damage to theabove described equipment. Capacitance sensing circuit 14 is capable ofmeasuring small changes (15-30 femtoFarads) in capacitance. Since anobject 12 changes capacitance as object 12 moves, raising of arms,crossing of legs, finger wiggling, toe wiggling, and torso motion areall detectable.

Capacitance sensing circuit 14 uses charge transfer technology tomeasure the capacitance of object 12 connected to circuit 14. Conductivestrap 16 is used, along with circuit 14 to measure an effective nominalcapacitance of object 12. Capacitance sensing circuit 14 is manually orautomatically re-calibrated for new nominal capacitive loads, such as,for example a different object 12. The re-calibration process changesthe nominal capacitance about which small changes, such as the movementof object 12 described above, are detected. Re-calibration allows system10 to accommodate objects 12 of different sizes, shapes, clothing, andbody hair, for example. Re-calibration also can take into account theenvironment object 12 is placed, such as temperature and relativehumidity.

FIG. 2 shows an alternative embodiment to the system shown in FIG. 1employing an alternative method for the measurement of capacitance.Non-conductive surface 22 and conductive surface 24 are as describedabove, however, a conductive mat 40 is placed on top of non-conductivesurface 22 and is electrically connected to capacitive sensing circuit14. The measured capacitance is based upon an amount of object 12actually touching conductive mat 40, as well as movement of object 12,such as raising of arms, crossing of legs, finger wiggling, toewiggling, and torso motion. For instance, the measured capacitance of ahuman body laying supine on mat 40 will be greater than the measuredcapacitance of a human body laying supine on mat 40 with both legs bentand the soles of the feet resting flat on mat 40. As object 12 moves andless of the body is touching mat 40, the measured capacitance willdecrease. The larger the surface area touching mat 40, the higher thecapacitance.

The embodiments shown in FIGS. 1 and 2 demonstrate, for example, how ahuman body, can be used as a detector for a capacitive sensing circuit.Measured capacitance depends on the location of objects relative to thebody, and other objects or persons near the subject being used as adetector can be detected. Using a subject as a detector may be idealwhen there is the potential for a number of moving components to makecontact with the subject, a sensor being installed on every movingcomponent being unfeasible.

FIG. 3 is a diagram showing one embodiment of a capacitance basedproximity sensor 50 used in systems where the subject is not used as thedetector. Sensor 50 includes a sensing surface 52 which is made of athin film of conducting material mounted on a front side 54 ofnon-conductive backing material 56. A backing surface 58 of electricallygrounded thin film conducting material mounted on a back side 60 ofnon-conductive backing material 56 completes the sensor. As statedabove, backing surface 58 is connected to an electrical ground 62.Sensing surface 52 is electrically connected to a capacitive sensingcircuit 64 and as shown in FIG. 3, may be configured to be of a sizesmaller in surface area than that of backing material 56.

FIG. 4 is a diagram showing an alternative embodiment of a capacitancebased proximity sensor 70. Sensor 70 is cylindrically shaped andconsists of sensing surfaces 72, 74 and 76 of the thin film electricallyconductive material. Sensing surface 72 covers an outer surface ofsensor 70 and sensing surfaces 74 and 76 cover end surfaces of thecylinder, a top surface and a bottom surface respectively.Non-conductive backing material 78 is the “body” of the cylinder, givingsensor 70 strength and a surface for the mounting of surfaces 72, 74 and76 which are electrically connected to a capacitive sensing circuit. Abacking surface (not shown) is electrically connected to ground. Inanother embodiment, the backing surface is not utilized by sensor 70.

FIG. 5 is a diagram of one embodiment of a proximity sensor 90configured to shape the sensing field. Sensor 90, in the embodimentshown in FIG. 5 consists of an outer sensing surface 92, a centralsensing surface 94, and an inner sensing surface 96. Outer sensingsurface 92, central sensing surface 94 and inner sensing surface 96 areelectrically connected with conductive strips 98 to form an electricallycontinuous circuit and are mounted on non-conductive backing material100. In one exemplary embodiment, sensor 90 has sensing surfacedimensions where inner sensing surface 96 has a dimension of 3 cm×3 cm,a space of 3 cm separates inner sensing surface 96 from an innercircumference 102 of central sensing surface 94 which is 3 cm wide.Another 3 cm gap in sensing material separates an outer circumference104 of central sensing surface 94 from an inner circumference 106 ofouter sensing surface 92. Outer sensing surface 92 is 3 cm in width. Inthe exemplary embodiment, backing material 100 is fabricated from mylar,which is virtually invisible to x-ray radiation. The thickness of themylar backing depends on mechanical strength requirements of anapplication. The sensing surfaces of sensor 90 are variable in size andin number in order to shape the sensing field of sensor 90 and areconnected to a capacitive sensing circuit 108. In the exemplaryembodiment, sensing surfaces 92, 94 and 96 of sensor 90 are fabricatedfrom 3 um thick aluminum foil and bonded to the mylar. In anotherembodiment, aluminum plates are bonded to the mylar. To be invisible toa vascular spectrum, surfaces 92, 94 and 96 are fabricated from aluminumfoil/plates less than 5 um in thickness. In a further embodiment,sensing surfaces 92, 94 and 96 are fabricated from copper. In a stillfurther embodiment, sensing surfaces 92, 94 and 96 are fabricated fromtin.

FIG. 6 is a diagram of an alternative circular embodiment of a proximitysensor configured to shape the sensing field. Sensor 110, in theembodiment shown in FIG. 6 consists of an outer sensing surface 112, acentral sensing surface 114, and an inner sensing surface 116. Outersensing surface 112 and central sensing surface 114 are ring shaped.Inner sensing surface 116 is circularly shaped. Outer sensing surface112, central sensing surface 114, and inner sensing surface 116 areelectrically connected with conductive strips 118 to form anelectrically continuous circuit and are mounted on non-conductivebacking material 120. In one exemplary embodiment, sensor 110 hassensing surface dimensions where inner sensing surface 116 has adiameter of 3 cm. A ring shaped space 122 that is 3 cm wide separatesinner circular sensing surface 116 from central sensing surface 114.Central circular sensing surface 114 has a inner ring 124 and an outerring 126. Inner ring 124 has a diameter of 9 cm and outer ring 126 has adiameter of 12 cm, such that central sensing surface 114 has a sensorring area of 3 cm in diameter. Another 3 cm wide ring shaped space 128in sensing material separates central sensing surface 114 from outersensing surface 112. Outer sensing surface 112 has an inner ring 130 andan outer ring 132. Inner ring 130 has a diameter of 21 cm and outer ring132 has a diameter of 27 cm, such that outer sensing surface 112 has asensor ring area of 3 cm in diameter. In the exemplary embodiment,backing material 120 is fabricated from mylar, which is virtuallyinvisible to x-ray radiation. The thickness of the mylar backing dependson mechanical strength requirements of an application. The sensingsurfaces of sensor 110 are variable in size and in number in order toshape the sensing field of sensor 110 and are connected to a capacitivesensing circuit 133. In the exemplary embodiment, sensing surfaces 112,114 and 116 of sensor 110 are fabricated from 3 um thick aluminum foiland bonded to the mylar. In another embodiment, aluminum plates arebonded to the mylar. To be invisible to a vascular spectrum, surfaces112, 114 and 116 are fabricated from aluminum foil/plates less than 5 umin thickness. In a further embodiment, sensing surfaces 112, 114 and 116are fabricated from copper. In a still further embodiment, sensingsurfaces 112, 114 and 116 are fabricated from tin.

FIG. 7 is a diagram 134 of a sensing field for a capacitance basedproximity sensor where no shaping has been employed, for example, wherethe sensing surface is a solid rectangular or square thin-filmconductor. Such a sensor is able to detect capacitive changesomni-directionally. The sensor which produces the type of field shown inFIG. 7 is more sensitive to objects which approach the sensor along anx=0 axis 136 and less sensitive to objects approaching along a y=0 axis138. Objects moving along axis 136 are detected more quickly and from afarther distance. This non-uniform sensitivity is not particularlydesirable.

FIG. 8 is a diagram of a sensing field 140 where the sensing surface hasbeen shaped using sensor 90, as described above and shown in FIG. 5. Inone embodiment, annular surfaces 92, 94, and 96 are optimized to flattenthe field at a 5 cm distance. The field shown is more uniform comparedto the field shown in FIG. 8, and is illustrative of an ability tocustomize field shaping by using segmented sensing surfaces. Thecircular construction used for field shaping can be extended to circularand cylindrical geometries (shown in FIG. 6).

FIG. 9 is a diagram of a medical imaging system 160 using capacitivebased proximity sensors 162 electrically connected to capacitive sensingcircuits 164. Capacitive sensors 162 also provide a non-contact methodof measuring the relative capacitance of a human body covered in paper,plastic and clothing. Circuits 164 provide data to a control system 166regarding position and orientation of component 168 relative to 170.Component 168 in one embodiment is a radiation source. Component 168 inan alternative embodiment is a detector. Component 168 in a furtheralternate embodiment is a sensor. Component 168 in a still furtherembodiment is a nuclear medicine imaging source. Component 168 inanother embodiment is a laser source. Component 168 in yet anotherembodiment is a component of a medical system, e.g., computer aidedtomography (CAT), magnetic resonance imaging (MRI), computed tomography(CT), digital fluoroscopy, positron emission tomography (PET), positronemission transaxial tomography (PETT), and mammography. Although notshown in the figure, system 166 controls elevation, longitudinalmovement and horizontal orientation of component 168. By using acapacitive based proximity approach, system 166 is configurable tofollow the contours of an object, such as a body 170. System 166 canthen be programmed to optimize the trajectory of component 168 relativeto the object 170. In an exemplary embodiment, system 166 reducesexposures to radiation to body 170 compared to known systems, whicheither do not change the radiation source, detector elevations, oremploy sensing devices, and which require a touching of body 170 beforea control system adjusts movement of component 168.

FIG. 10 is an illustration of an irregularly shaped apparatus 180 withan outer surface 182 covered with sensing material 184. Sensing material184 is fabricated from a thin-film conducting material, e.g., aluminum,copper or tin. In an exemplary embodiment, thin-film sheets of copperfoil are joined together with conductive epoxy 186. In one embodiment,the copper foil is 25 um in thickness. In an alternative embodiment, thethin-film sheets are fabricated by “spray depositing” a film ofconductive material, e.g., tin, to a backing surface. Sensing material184 is bonded to a backing surface (not shown). In an alternativeembodiment, apparatus 180 is configured to take any form and shape andis not limited to a certain size range. In addition, sensing material184 is electrically coupled to a capacitive sensing circuit (not shown).Apparatus 180 has one sensing zone 188. In an alternative embodiment,sensing material 184 has a plurality of sensing zones 188. Sensing zone188 is capable of measuring changes in capacitance, e.g., 15-30femtoFarads. In one embodiment, sensing zone 188 is optimized fordetecting predetermined objects at a specified distance. In analternative embodiment, apparatus 180 includes a plurality of sensingzones, each sensing zone optimized to detect a predetermined object at aspecified distance.

While the invention has been described in terms of various specificembodiments, those skilled in the art will recognize that the inventioncan be practiced with modification within the spirit and scope of theclaims.

1. A method for detecting motion of an object using a capacitance based sensing and control system, said method comprising: sensing a presence of the object based on measured capacitance between a sensor and the object including measuring charge transfer to determine the relative capacitance of the object; sensing a change in capacitance of the object; and adjusting operation of the control system based upon said sensed capacitance change.
 2. A method according to claim 1, further comprising recalibrating the control system when a new, nominal capacitive load is detected or at a user's discretion.
 3. A method according to claim 1, wherein sensing a change in capacitance further comprises sensing changes in the geometry of the object.
 4. A method according to claim 1, wherein sensing a change in capacitance further comprises sensing proximity of the object to other objects.
 5. A method in accordance with claim 1, wherein the object is a human body.
 6. A capacitance based proximity sensor comprising: a sensing surface of thin film conducting material; and a non-conducting backing material comprising a front side and a back side, said sensing surface mounted on said front side, wherein said sensing surface is configured to be of a smaller surface area than said backing material.
 7. A sensor according to claim 6, wherein said sensing surface is electrically coupled to a capacitance sensing circuit.
 8. A sensor according to claim 6, wherein said sensor further comprises an optional backing surface of conducting material upon which said back side of said non-conducting backing material is mounted.
 9. A sensor according to claim 8, wherein said optional backing surface is electrically coupled to a circuit ground.
 10. A sensor according to claim 6, wherein said sensor is configured to be cylindrically shaped.
 11. A sensor according to claim 10, wherein said sensing material is configured to cover an outer surface and both end surfaces of the cylinder.
 12. A sensor according to claim 6, wherein said sensor is rectangularly shaped.
 13. A sensor according to claim 12, wherein said rectangularly shaped sensor is approximately 20 cm in both length and width.
 14. A sensor according to claim 12, wherein said sensing surface comprises a plurality of electrically connected rectangular shaped conductors, said rectangular conductors each having an inner dimension and an outer dimension.
 15. A sensor according to claim 14, comprising three rectangular shaped conductors.
 16. A sensor according to claim 15, wherein a first rectangularly shaped conductor comprises an inner dimension of 0 cm and an outer dimension of 1.5 cm, a second rectangularly shaped conductor comprises an inner dimension of 4.5 cm and an outer dimension of 7.5 cm, and a third rectangularly shaped conductor comprises an inner dimension of 10.5 cm and an outer dimension of 14.75 cm.
 17. A sensor according to claim 6, wherein said sensor is circularly shaped.
 18. A sensor according to claim 17, wherein said circularly shaped sensor is approximately 21 cm in diameter.
 19. A sensor according to claim 17, wherein said sensing surface comprises a plurality of electrically connected circularly shaped conductors, said circular conductors each having an inner dimension and an outer dimension.
 20. A sensor according to claim 19, comprising three circular shaped conductors.
 21. A sensor according to claim 20, wherein a first circularly shaped conductor comprises a diameter of 3 cm, a second ring shaped conductor comprises an inner diameter of 9 cm and an outer diameter of 15 cm, and a third ring shaped conductor comprises an inner diameter of 21 cm and an outer diameter of 27 cm.
 22. A sensor according to claim 6, wherein said sensor is irregularly shaped.
 23. A sensor according to claim 22, wherein said irregularly shaped sensor is approximately 21 cm in length and width.
 24. A sensor according to claim 22, wherein said sensing surface comprises a plurality of electrically connected irregularly shaped conductors, said irregularly shaped conductors each having an inner dimension and an outer dimension.
 25. A sensor according to claim 24, comprising three irregularly shaped conductors.
 26. A sensor according to claim 25, wherein a first irregularly shaped conductor comprises a length and width of 3 cm, a second irregularly ring shaped conductor comprises an inner length and width of 9 cm and an outer length and width of 15 cm, and a third irregularly ring shaped conductor comprises an inner length and width of 21 cm and an outer length and width of 27 cm.
 27. An apparatus comprising: a sensing surface of thin film conducting material, said sensing surface configured to have a plurality of sensing zones conflaured to partially cover an outer surface of said apparatus; a non-conducting backing material comprising a front side and a back side, and said sensing surface mounted on said non-conducting backing.
 28. An apparatus in accordance with claim 27, wherein said apparatus configured to be at least one of a sensor and a detector.
 29. An apparatus in accordance with claim 27, wherein said backing surface is electrically coupled to a circuit ground.
 30. An apparatus in accordance with claim 27, wherein said apparatus is configured to be cylindrically shaped.
 31. An apparatus in accordance with claim 27, wherein said sensing material is configured to cover an outer surface of said apparatus.
 32. An apparatus in accordance with claim 27, wherein said sensing zones configured to be electrically coupled to a capacitive sensing circuit.
 33. An apparatus in accordance with claim 27, wherein said sensing zones configured to be spaced equidistant from one another.
 34. An apparatus in accordance with claim 27, wherein said sensing surface is electrically coupled to a capacitive sensing circuit.
 35. An apparatus in accordance with claim 34, wherein said capacitive sensing circuit configured to measure a nominal capacitance at least up to 2500 pF.
 36. An apparatus in accordance with claim 27, wherein said apparatus is configured to be an irregular shape.
 37. An apparatus in accordance with claim 36, wherein said apparatus is configured to be an irregular shape including a angled front side, a flat back side, an open top side, a convex first side, a convex second side offset from said first side.
 38. An apparatus in accordance with claim 27, wherein said apparatus is configured to be rectangularly shaped.
 39. An apparatus in accordance with claim 38, wherein said sensing material is configured to be of a smaller surface area than said backing material.
 40. An apparatus in accordance with claim 27, wherein said sensing surface configured as a single sensing zone.
 41. An apparatus in accordance with claim 40, wherein said sensing zone configured to be electrically coupled to a capacitive sensing circuit.
 42. A method for detecting motion of an object using a capacitance based sensing and control system, said method comprising: sensing a presence of the object based on measured capacitance between a sensor and the object; sensing a change in capacitance of the object; adjusting operation of the control system based upon said sensed capacitance change; and recalibrating the control system when a new, nominal capacitive load is detected or at a user's discretion. 